The present invention relates generally to induction-type linear position detector devices, and more particularly to an induction-type linear position detector device which, on the basis of excitation by a single-phase A.C. signal, induces and outputs A.C. signals indicative of plural-phase amplitude function characteristics responsive to a linear position to be detected.
Among various examples of the conventional induction-type linear position detector devices are differential transformers. Generally, in the differential transformers, a single primary winding is excited by a single-phase input signal in such a manner that there occurs, at individual locations of two differentially-connected secondary windings, reluctance varying in response to a varying linear position of an iron core moving with an object of detection, so that the voltage amplitude level of a resultant single-phase inductive A.C. output signal indicates the linear position of the iron core. These differential transformers are capable of detecting a linear position only over a limited range where the induced voltage value shows linearity with respect to the linear position, at and around the locations of the two secondary windings provided in such a manner that the induced voltage varies in a differential manner, and the function of variation in the induced voltage value relative to the linear position does not change over a periodic function (e.g., a trigometric function such as a sine function). Therefore, the only way to expand the detection range is to increase the lengths of the windings and iron core, but this approach naturally has a limitation and also would undesirably result in an increased size of the device. In addition, it is impossible to yield an output indicative of an electrical phase that correlates to a current linear position of the object of detection. Further, because the voltage amplitude level of the induced voltage is easily influenced by various environmental variations such as temperature changes, the known linear position detector devices could not provide a sufficient detecting accuracy. Such induction-type linear detectors are known, for example, from U.S. Pat. Nos. 2,469,137 and 3,242,472.
Further, from U.S. Pat. Nos. 4,297,698, 4,556,886, etc., these phase-shift-based induction-type linear position detector devices are known which output an A.C. signal having an electrical phase angle correlating to a current linear position of an object of detection. In these phase-shift-based induction-type linear position detector devices, a plurality of (e.g., two) primary windings, displaced from each other with respect to a direction of linear movement of an iron core moving in response to a varying position of the object of detection, are excited by two-phase A.C. signals with different electrical phases (e.g., sin .omega.t and cos .omega.t), so that resultant induced signals in secondary windings are combined to provide a single secondary output signal. The electrical phase difference, of the secondary output signal, from the exciting A.C. signals represents a linear position of the iron core moving in response to a varying position of the object of detection. Further, according to the disclosure of U.S. Pat. No. 4,556,886, a plurality of iron cores are provided at a predetermined pitch, so as to permit a detection of linear positions over a substantially wider range than an extent where primary and secondary windings are located.
However, because A.C. signals of at least two phases (e.g., sin .omega.t and cos .omega.t) have to be supplied for the necessary excitation, the above-discussed conventional phase-shift-based induction-type linear position detector devices would require an exciting circuit of complicated structure, although they do have many advantages over the differential transformers. These detector devices also present the problem that errors would occur in the electrical phase of the secondary output signal when impedance of the primary and secondary windings varies due to temperature changes etc. Further, in the devices where the iron cores are provided with a predetermined pitch so as to permit a detection of linear positions over a substantially wider range than the region where the primary and secondary windings are located, the range where the primary and secondary windings are located must be longer than one pitch length of the iron cores, which would result in an increased size of the entire winding assembly and thus pose a significant limitation to a user's increasing demand for miniaturization of the detector devices. That is, where one pitch length of the iron cores is assumed to be "P" in a four-phase-based detector device, the pitch length of the windings of the individual phases must be at least "3P/4" and four times that pitch length, i.e., "4.times.(3P/4)", would be necessary, as a whole, as the range where the primary and secondary windings are located. Thus, the winding assembly had to be provided at least along a length corresponding to three pitch lengths of the movable iron cores.